The task of replacing a tooth is conventionally made of two separate steps. The first step is to measure the shape and color shade of a tooth to be replaced and the second step is to make a duplicate of that tooth according to the measurements taken in the first step.
In the first step, while the shape information can be acquired with molding technique, the measurement of the color shade and translucency of the tooth proves to be more challenging.
The quality of the dental prosthesis cannot be better than the data that serves to model the tooth. The precision of that model depends on several factors, like the quality of the illumination, the data acquisition by measuring and the processing of those data.
The oldest and simplest way of determining the color shade of an object like a tooth is to compare visually the object with a chart of color shades. The results obtained with that method are however not very good because of the subjectivity of the human eye. Furthermore, the illumination of the tooth and of the chart may cause inappropriate color shade choices.
A quantitative method can be used to obtain a minimum of precision and of reproductability in the measurement of the color shade of an object. Such quantitative methods can be classified by the type of illumination used, the measurement technique, the data processing and the comparison between the finished product and the original object.
The illumination is usually done by using fiber optics or a fiber optic bundle to illuminate the surface of the object to be measured. It is advantageous to control the illumination of the object since the characteristics of the illumination method may be taken into account during the data processing. Diffuse light provides a simple means to control illumination. An example can be found in the U.S. Pat. No. 5,383,020 issued in Jan. 17, 1995 and naming Vieillefosse as the inventor.
Integrating spheres are a known technique to achieve a uniform diffuse light source. Such a technique of illuminating a tooth is described by O'Brien in the U.S. Pat. No. 5,759,030, issued on Nov. 21, 1989. This type of illumination is useful for measurement of matte surfaces. A drawback of that technique, or of any other technique that produces diffuse light, is apparent when it is used to illuminate glossy material. The desired signal is then confounded with a specular reflection component. The classification of tooth shades requires that the illumination be known with a precision of at least one percent everywhere on the tooth surface.
Different measurement techniques are presently used to quantize the reflected light coming from an illuminated object. These techniques usually consist in a spectral decomposition of the reflected light from a selected area of the object surface.
Vieillefosse et al. describe, in U.S. Pat. No. 5,428,450, issued on Jun. 27, 1995, a method for determining the color of an object by decomposing the light with an optical system consisting of achromatic doublets and by analysing the light by means of interference filters and photo detectors. In the above mentioned O'Brien's patent, there is described a device for decomposing the light, comprising a spectrophotometer.
A drawback of both Vieillefosse et al. and O'Brien's methods is that the selected area of the object surface is seen as if it was uniform or a point. The spatial differences are not detected by these methods and thus can not be reproduced in the duplicated teeth. Another drawback of Viellefosse's method is that the wavelength spectrum is limited to only five wavelengths.
Another measurement technique is taught by Murljacic in his U.S. Pat. No. 5,766,006, issued on Jun. 16, 1998. In this document, Murljacic describes a tooth shade analyser system using a camera to capture a digital color image of a tooth. The tooth image includes an RGB chromaticity representation that is scanned and compared pixel by pixel with several tooth shades stored in a memory of the system.
A drawback of Murljacic's system is that the scanning is performed without controlling the illumination therefore decreasing the reproductability of the color comparison.
Several methods are known and used to convert the spectral decomposition or the data collected from a selected area into a single measurement that corresponds to the color perception of the human eye. The objective is to quantize the data and also to correct them as to be able to recreate the proper colors of the original model as the human eye perceives them. It is also important to be able to quantize the translucency of the materials.
A method of processing data is described by O'Brien. It consists in converting the measurements to tristimulus values, after calibration on a chip, and comparing to known tabulated values. The tristimulus value conversions are performed under a given illumination, represented by tabulated values determined to represent most appropriately power frequency distribution of an incandescent lamp. A problem of that method is that it does not process images obtained by a properly color-calibrated measurement device.
A general drawback of the prior art lies in that the notion of comparison between the measured tooth and a duplicate is limited to a single point to point comparison. Thus, area defects cannot be detected where no measurement has been taken.